Greetings:
The May 2012 edition of IEEE's
Spectrum
magazine has an interesting article discussing the
frustrating effort to establish a new method for
determining the standard kilogram. For 125 years,
the master standard has been a platinum-iridium
alloy cylinder about the size of a plumb. It is
stored in a vault, under three concentric Bell jars.
Every three years, the master standard is cleaned,
and every decade or so, the standard is compared
to other country's identical standards to determine
whether they all still measure the same mass. The
problem is that aside from the need to maintain
a physical block, the cumbersome process of comparative
measurement introduces perturbations and errors
in the results. The master standard has apparently
lost 55 micrograms since the first measurements,
and nobody knows where they went. That is not a
lot of mass compared to a kilogram, but with the
precision of measurements made by laboratory and
even commercial instruments and products, an uncertainty
in the eighth decimal point can be a serious handicap.
The article is only four pages long, but it
covers a lot of ground, including the current proposals
for the new master standard that does not involve
storing a specific mass.
The Kilogram, Reinvented
http://spectrum.ieee.org/consumer-electronics/standards/the-kilogram-reinvented/0
